It means process of requesting an uncustomized page instance by using a page template loaded into memory from the file system of the front-end Web server.
These Ghosted pages are pulled from the cache at runtime and therefore it will increase the scalability from the system.
All uncustomized pages are reused across all the sites and there is no unnecessary data storage or retrieval.
Ghosting eliminates the need to transfer the contents of a page definition file from the SQL Server computer with the content database to the front-end Web server computer
For example, the default home page is a ghosted page. Any web part pages created via New Web Part Page user interface also ghosted.
It means that the site has been customized. When you customize a site in SharePoint Designer, or you add custom fields to a Document Library, or create sites using that template; then the changes that you made are stored in the database as a difference and that is referred to as Unghosted.
A request for a Customized (Un-Ghosted) page returns a response which is a combination of data from both the file system and the database.
All Unghosted sites are based on a Site Definition.